A typical prior art "embedded-lens" retroreflective sheeting, as illustrated in FIG. 1 of the attached drawing, comprises a monolayer of microspheres 11 embedded between transparent bonding and spacing layers 12 and 13; a specularly reflective layer 14, typically aluminum vapor-deposited on the spacing layer 13; a layer of adhesive 15 covering the reflective layer; and a transparent top layer 16, which forms the exterior front surface of the sheeting. Light rays incident on the sheeting travel through the layers 16 and 12 to the microspheres 11, which act as lenses focusing the incident light approximately onto the appropriately spaced specularly reflective layer 14. Thereupon the light rays are reflected back out of the sheeting along substantially the same path as they traveled to the sheeting.
Embedded-lens sheeting as described has the advantage that, because the microspheres are embedded within the sheeting, incident light rays are focused onto the specularly reflective layer irrespective of whether the front of the sheeting is wet or dry. Such sheeting was first taught in Palmquist et al, U.S. Pat. No. 2,407,680, and has been sold commercially for many years in large volume and to the general satisfaction of its users. Despite that general utility, however, there has been a desire for improvement in certain properties of the sheeting, one of which is in the angles at which the sheeting will retroreflect brightly. Retroreflection is generally at a maximum for light that is approximately perpendicular to the sheeting (i.e., has an incidence angle of 0.degree.) and declines as the incident light slants away from the perpendicular (i.e., increases in incidence angle). Such a decline in retroreflectivity can be a disadvantage, for example, by limiting the length of time that a retroreflective traffic-control sign is seen at night as a vehicle passes by, or the length of time a retroreflective license plate will be seen as two vehicles meet on the highway.